Estimation of frequency offset between a base station and mobile terminal

ABSTRACT

A method and apparatus for frequency offset estimation exploits the differences in reference symbol timing for different channels to resolve ambiguities in the frequency offset estimation. Based on the initial frequency offset estimates, a hypothesis table is constructed providing hypothesized frequency offsets for each channel for a plurality of possible offset regions. An error metric for each offset region is calculated based on the difference of the hypothesized frequency offsets. The set of hypothesized frequency offsets that minimize the error metric is selected as the final frequency offset estimates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/170,930, filed on Jun. 28, 2011, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

The present invention relates generally to frequency correction in amobile communication terminal and, more particularly, to the estimationof the frequency offset between a base station and mobile terminal in amobile communication network.

In mobile telecommunication systems, there is typically frequency offsetbetween the transmitter and the receiver. The frequency offset can becaused by oscillator mismatch in the transmitter and the receiver and/orDoppler shift. Under certain channel conditions, the frequency offsetdue to Doppler shift can be quite large, i.e. greater that 1 kHz. Onescenario where large frequency offsets are expected is the high speedtrain (HST) scenario as defined in the Third Generation PartnershipProject (3GPP) specification TS 36.104 where the user is traveling on ahigh speed train. Due to large Doppler shift in the HST scenario, thefrequency offset can be ±2 kHz for the Evolved Universal TerrestrialRadio Access (E-UTRA) operating band 7 frequency. For the mobilecommunication systems to work properly, a frequency offset of thismagnitude must be estimated and corrected.

Simple and effective algorithms exist to estimate the frequency offset.One common approach estimates the frequency offset based on the phasedifferences of reference symbols (or pilot symbols). The frequencyoffset f_(offset) is estimated as the phase difference ΔΦ divided by thetime interval Δt of the pilot symbols, and may be given by:

$\begin{matrix}{f_{offset} = {\frac{\Delta\Phi}{2\pi \times \Delta\; t}.}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$Because the observable phase difference is limited to an absolute valueless than π, the time interval of the pilot symbols determines themaximum frequency offset range that can be estimated. For the PhysicalUplink Shared Channel (PUSCH) in LTE systems, the time interval of thereference symbols is 0.5 ms. The maximum frequency offset that can beestimated from PUSCH reference symbols is ±1000 Hz. For the PhysicalUplink Control Channel (PUCCH) using format 2/2a/2b, the time intervalfor the reference symbols is approximately 4/14 ms. The maximumfrequency offset that can be estimated from PUCCH reference symbols isthus ±1750 Hz. If the frequency offset is beyond these ranges, the phasedifferences of the reference symbols will “wrap-around” π or −π. The“wrap-around” frequencies corresponding to the phase differences of ±πare ±1000 Hz for PUSCH and ±1750 Hz for PUCCH.

The wrap-around effect creates ambiguity in estimating the frequencyoffset. For example, a frequency offset of −1750 Hz for the PUSCH willcause phase rotation and end up at point A in the complex plane as shownin FIG. 1. Similarly, a frequency offset of +250 Hz will cause a phaserotation of

$\frac{\pi}{4}$sand end up at the same point in the complex plane. In this example, the−1750 Hz and +250 Hz frequency offsets are indistinguishable. Thepotential for large frequency offsets beyond the resolvable range thuscreates an ambiguity that needs to be resolved to determine the correctfrequency offset.

A method for increasing the resolvable range of frequency offsets isdescribed in the patent application WO 2010/060732 “Frequency OffsetEstimation” combining two frequency offset estimates on the samereceived signal. In this disclosure, the two estimates are calculatedfrom pairs of received symbols with different time difference betweenthe first and the last received symbol. However, for each PUSCH andPUCCH format 2/2a/2b there is only one time difference between thereference symbols located in the same part of the spectrum so it is notpossible to resolve the ambiguity from a single received signal.Furthermore, the PUSCH and PUCCH can not be scheduled in the samesubframe for the same user and one channel (PUSCH for example) may bescheduled more frequently than the other (PUCCH for example). Thus,there will be instances where there are no fresh raw estimates from bothchannels. In these cases, a channel has to resolve the ambiguity withits own single raw estimate.

Therefore, new techniques are needed for extending the resolvable rangeof frequency offsets.

SUMMARY

The present invention provides methods and apparatus for estimating thefrequency offset between a transmitter and a receiver. The presentinvention exploits the differences in reference symbol timing fordifferent channels to resolve ambiguities in the frequency offsetestimation. Based on the initial frequency offset estimates, ahypothesis table is constructed providing hypothesized frequency offsetsfor each channel for a plurality of possible offset regions. An errormetric for each offset region is calculated based on the differencebetween the hypothesized frequency offsets. The set of hypothesizedfrequency offsets that minimizes the error metric is selected as thefinal frequency offset estimates.

Exemplary embodiments comprise a method implemented in a receiving nodeof a wireless network for determining a frequency offset between atransmitting station and a receiving station. In one exemplary method,raw frequency offset estimates are generated for first and secondchannels having different resolvable ranges. A first hypothesizedfrequency offset is computed for the first channel and a secondhypothesized frequency offset is computed for the second channel foreach one of two or more offset regions within an offset range ofinterest. For each offset region, an error metric is computed as afunction of the corresponding hypothesized frequency offsets for thefirst and second channels. The hypothesized frequency offsetscorresponding to the offset region having the lowest error metric areselected as final frequency offset estimates.

Other embodiments of the invention comprise a receiving node in wirelessnetwork. The receiving node comprises a receiver front end for receivingsignals from a transmitting node over a wireless channel and aprocessing circuit to estimate a frequency offset between the receivingnode and transmitting node. The processing circuit is configured togenerate raw frequency offset estimates for first and second channelshaving different resolvable ranges. The processing circuit computes,from the raw frequency offset estimates, a first hypothesized frequencyoffset for the first channel and a second hypothesized frequency offsetfor the second channel for each one of two or more offset regions withina region of interest. The processing circuit then computes an errormetric for each offset region as a function of the correspondinghypothesized frequency offsets for the first and second channels.Finally, the processing circuit selects, as final frequency offsetestimates, the hypothesized frequency offsets corresponding to theoffset region having the lowest error metric.

In some embodiments, the frequency offset selected in a first timeinterval may be used to select the frequency offset in a subsequent timeinterval. More particularly, the processing circuit may select ahypothesized frequency offset in a second time interval that is theclosest to the selected frequency offset in the first time interval.

The frequency offset estimation techniques herein described expand theresolvable frequency offset range, which allows for more accurateestimation of the frequency offset with only a slight increase incomputing resources. More accurate frequency offset estimation paves theway for proper frequency offset compensation, thus ensuring a higherlevel of throughput performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the aliasing problem in frequency offset estimation.

FIG. 2 illustrates an exemplary base station according to one embodimentof the invention including a frequency offset estimator.

FIG. 3 illustrates the main functional components of the frequencyoffset estimator.

FIG. 4 illustrates how joint estimation extends the resolvable frequencyoffset range.

FIG. 5 illustrates an exemplary method according to one embodiment.

DETAILED DESCRIPTION

FIG. 2 illustrates an exemplary base station 10 according to oneexemplary embodiment. Base station 10 is configured to operate accordingto the Long Term Evolution (LTE) standard by the 3rd GenerationPartnership Project (3GPP). Those skilled in the art will appreciatethat the principals described herein may be applied to mobilecommunication networks based on other communication standards including,without limitation, Wideband Co-Division Multiple Access (WCDMA)systems, WiMax systems, and wireless local area networks (WLAN).

Base station 10 comprises a transceiver circuit 20 and processingcircuit 50. Transceiver circuit 20 comprises the radio equipment forcommunicating over the air interface with mobile terminals via antenna22. The transceiver circuit 20 comprises a receiver front end 30 andtransmitter front end 40. Receiver front end 30 downconverts thereceived signal to baseband frequency, amplifies and filters thereceived signal, and converts the received signal to digital form forinput to the processing circuit 50. Transmitter front end 40 convertssignals supplied by the processing circuit 50 to analog form, filtersand amplifies the signal, and modulates the signal onto an RF carrierfor transmission to the mobile terminal via antenna 22.

The processing circuit 50 performs the digital signal processing fortransmitted and received signals, and controls the operation of the basestation 10 according to the applicable communication standard.Processing circuit 50 demodulates and decodes the received signalssupplied by the receiver front end 30. The processing circuit 50 alsoencodes and modulates signals to be transmitted to the mobile terminalby the transmitter front end 40. Processing circuit 50 includes ascheduler 60 and a frequency offset estimator 100. The frequency offsetestimator 100 estimates a frequency offset between the transmitter atthe mobile terminal and the receiver at the base station 10. Thefrequency offset may be due to variances in the oscillator and Dopplershift. The frequency offset estimator 100 applies frequency correctionto correct for the estimated offset. The scheduler 60 handles radioresource management and schedules transmissions to the mobile terminalon the downlink channels, and transmissions from the mobile terminal onthe uplink channels. Those skilled in the art will appreciate that theprocessing circuit 50 performs many other functions which are notessential to the understanding of the present invention.

FIG. 3 illustrates the main functional components of a frequency offsetestimator 100 according to one exemplary embodiment. Those skilled inthe art will appreciate that the functional components shown in FIG. 3represent logical functions of the frequency offset estimator 100,rather than physical components. The logical components may beimplemented in a microprocessor, application specific integrated circuit(ASIC), or other digital signal processor.

The frequency offset estimator 100 comprises first and second phasedetectors 110 and 120, respectively. A first received signal received ona first channel is input to the first phase detector 110, and a secondsignal received on a second channel is input to the second phasedetector 120. In one exemplary embodiment, the first signal is receivedon the physical uplink shared channel (PUSCH) in an LTE system. Thesecond signal is received on the physical uplink control channel (PUCCH)in an LTE system. The first phase detector 110 generates an initialfrequency offset estimate based on the phase difference between PUSCHreference symbols. Similarly, the second phase detector 120 determinesan initial frequency offset estimate for the PUCCH based on the phasedifference between PUCCH reference symbols. The initial frequencyoffsets from the phase detectors 110, 120 are applied to a jointestimator 130. Joint estimator 130 resolves ambiguity in the initialfrequency offset estimates and generates the final frequency offsetestimates for the PUSCH and PUCCH, respectively. The ambiguity ininitial frequency offset estimates is resolved using a joint estimationtechnique as herein described.

The joint estimator 130 exploits the difference in the periodicity ofthe reference symbols for the two channels considered. Continuing withthe LTE example, the time interval of the pilot symbols is 0.5 ms forthe PUSCH, which equates to a maximum resolvable frequency offset of±1000 Hz. For the PUCCH using format 2/2A/2B, the time interval of thereference symbols is approximately 4/14 milliseconds, which equates to aresolvable frequency offset of ±1750 Hz. Wrap-around occurs when theactual frequency offset exceeds the maximum resolvable frequency offset.For the PUSCH, the apparent phase shift for a frequency offset of −1750Hz is the same as a frequency offset of +250 Hz.

Joint estimator 130 considers the initial frequency offsets jointly toresolve the ambiguity of the initial frequency offsets. Under idealconditions, e.g., assuming that the frequency offset during PUSCHtransmission is the same as the frequency offset during PUCCHtransmission, the maximum resolvable frequency offset can be expanded to7000 Hz. FIG. 4 illustrates how joint estimation extends the resolvablefrequency offset range. When combining the PUSCH and PUCCH, the minimumchange interval will be 1/14 ms, which equals the greatest common factorbetween the 0.5 ms time interval for PUSCH and the 0.285 ms timeinterval for the PUCCH. The maximum resolvable frequency offset withoutaliasing is therefore 14,000/2, which equals 7000 Hz. In practicalapplications, the PUSCH and PUCCH transmissions happen at differenttimes and with time varying frequency offsets. Thus, the practicalmaximum resolvable frequency offset will be smaller than 7000 Hz.

The scheduler 60 at the base station 10 schedules PUCCH format 2 andPUSCH transmissions on a regular basis. The transmission intervals ofthe PUSCH and PUCCH may be different. Typically, the PUSCH has a shortertransmission interval than the PUCCH. The PUSCH and PUCCH provide twodifferent sets of reference symbols for frequency offset estimation.Also, as previously noted, the resolvable frequency offset range for thePUCCH and PUSCH are different due to the different reference symboltiming.

The frequency offset estimation procedure can be broken down into fourbasic steps. In the first step, initial frequency offset estimates aregenerated for the PUSCH and PUCCH, e.g., by the first and second phasedetectors 110, 120. The initial frequency offset estimates may bedetermined in a conventional manner by measuring the phase difference ofthe reference symbols. That is, the frequency offset for the PUSCH isdetermined by measuring the phase difference of the PUSCH referencesymbols and the initial frequency offset for the PUCCH is made bymeasuring the phase difference of the PUCCH reference symbols. Theinitial frequency estimates will fall within the respective resolvableranges for the PUSCH and PUCCH. Thus, the initial frequency offsetestimate for the PUSCH will be ±1000 Hz and the initial frequency offsetestimate for the PUCCH will be ±1750 Hz. The initial frequency offsetestimates may be averaged over time to improve the robustness againstnoise.

The second step of the frequency offset estimation procedure is theconstruction of a hypothesis table. The hypothesis table divides thefrequency offset range of [0,7000] Hz into 10 offset regions as shown inTable 1 below.

TABLE 1 Offset Regions Hypothesis PUSCH PUCCH absolute value Index ofwrap-around wrap-around frequency hypothesis number: number: offsetrange: regions: i_(h) n_(pusch) ^(wrap) n_(pucch) ^(wrap)

_(range) 0 0 0   (0, 1000) 1 1 0 (1000, 1750) 2 1 1 (1750, 2000) 3 2 1(2000, 3000) 4 3 1 (3000, 3500) 5 3 2 (3500, 4000) 6 4 2 (4000, 5000) 75 2 (5000, 5250) 8 5 3 (5250, 6000) 9 6 3 (6000, 7000)The boundaries between the offset regions are integer multiples of thewrapping thresholds for the PUSCH and/or PUCCH. As previously noted, thewrapping threshold for the PUSCH is ±1000 Hz and the wrapping thresholdfor the PUCCH is ±1750 Hz. Each offset region corresponds to apredetermined number of wraps, i.e. the wrap-around number, for thePUSCH and PUCCH. Table 1 gives the wrap-around number for each offsetregion for the PUSCH and PUCCH, where the combination of wrap-aroundnumbers for the PUSCH and PUCCH is unique in each offset region. Basedon the initial frequency offset estimates, the wrap-around numbers, andthe wrapping threshold, hypothesized frequency offsets for the PUSCH andPUCCH can be derived for each offset region

The third step of the frequency offset estimation procedure is tocompute an error metric for each offset region. Assuming that the actualfrequency offset for the PUSCH and PUCCH is the same, the hypothesizedfrequency offsets for the PUCCH and PUSCH in any given offset regionwould be equal. In practice, there is likely to be some variance betweenthe frequency offset estimates for the PUSCH and PUCCH. Accordingly,embodiments of the present invention use an error metric based on thedifference between the hypothesized frequency offsets for the PUSCH andPUCCH. In some embodiments, the error metric may also take into accounta difference between the hypothesized frequency offset for the PUSCH (orPUCCH) in the current transmission interval and the final frequencyoffset for the preceding transmission interval. The computation of theerror metrics is described in greater detail below.

The final step of the frequency offset estimation procedure is to selectfinal frequency offset estimates from the set of hypothesized estimates.In some embodiments, the final frequency offset estimates will be theset of hypothesized frequency offsets that minimize the error metric.

The PUSCH is normally scheduled more often than PUCCH. When there is nonew raw PUCCH frequency offset estimates, the PUSCH frequency offset ischosen from its hypothesis table with the one whose value is the closestto previous ambiguity resolved PUSCH frequency offset estimate.

An exemplary procedure for frequency offset estimation will now bedescribed in more detail. The description uses the followingdefinitions:

-   -   f^(actual): Actual frequency offset in Hz    -   f^(wrap) ^(—) ^(thresh): A positive number with a unit of Hz        denoting the wrap around threshold of frequency offset.        f_(pusch) ^(wrap) ^(—) ^(thresh)=1000 Hz,    -   f_(pucch) ^(wrap) ^(—) ^(thresh) =1750 Hz. This frequency offset        threshold corresponds to a phase shift of π during the        observation interval. If |f^(actual)|>f^(wrap) ^(—) ^(thresh),    -   the phase shift it causes during the observation interval will        be larger than π, i.e., a wrap-round has happened    -   n^(wrap): The number of wrap-around that will happen given a        frequency offset    -   {circumflex over (f)}: The raw frequency offset estimate.        Instantaneous frequency offset estimate in the range of        (−f^(wrap) ^(—) ^(thresh), 0) or (0, f^(wrap) ^(—) ^(thresh))    -   : Vector containing the hypothesized estimates of the actual        frequency offset based on the triplet: {circumflex over (f)},        f^(wrap) ^(—) ^(thresh) and hypothesis of n^(wrap)    -   {tilde over (f)}: The unwrapped value of raw frequency offset        chosen from {umlaut over (f)}    -   f′: Ambiguity resolved frequency offset estimate    -   f′^(k−1): Previous ambiguity resolved frequency offset estimate        The definitions apply to both the PUSCH and PUCCH, which are        distinguished by adding subscript _(pusch) or _(pucch).

The hypothesized frequency offset for the PUSCH and PUCCH for eachoffset region can be computed by first computing

_(tmp) from the raw frequency offset estimate according to:

_(tmp) ={circumflex over (f)}−sgn({circumflex over (f)})×mod(n ^(wrap),2)×2×f ^(wrap) ^(—) ^(thresh).   Eq. (2)Once

_(tmp) is known, the hypothesized frequency offset can be computedaccording to:

=

_(tmp) +sgn(

_(tmp))×└n ^(wrap)/2┘×2×f ^(wrap) ^(—) ^(thresh).   Eq. (3)

In PUSCH subframes where new PUCCH raw frequency offset estimates areavailable since the last PUSCH transmission, new hypothesized frequencyoffsets are computed for both the PUSCH and PUCCH. Hypothesizedfrequency offsets for the PUSCH are computed for an offset region i_(h)according to:

_(tmp) _(—) _(pusch) ={circumflex over (f)} _(pusch) −sgn({circumflexover (f)} _(pusch))×mod(n _(pusch) ^(wrap)(i _(h)), 2)×2×f _(pusch)^(wrap) ^(—) ^(thresh)   Eq. (4)

_(pusch)(i _(h))=

_(tmp) _(—) _(pusch) sgn(

_(tmp) _(—) _(pusch))×└n _(pusch) ^(wrap)(i _(h))/2┘×2×f _(pusch)^(wrap) ^(—) ^(thresh)   Eq. (5)

Hypothesized frequency offsets for the PUCCH are computed for eachoffset region i_(h) according to:

_(tmp) _(—) _(pucch) ={circumflex over (f)} _(pucch) −sgn({circumflexover (f)} _(pucch))×mod(n _(pucch) ^(wrap)(i _(h)), 2)×2×f _(pucch)^(wrap) ^(—) ^(thresh)   Eq. (6)

_(pucch)(i _(h))=

_(tmp) _(—) _(pucch) +sgn(

_(tmp) _(—) _(pucch))×└n _(pucch) ^(wrap)(i _(h))/2┘×2×f _(pucch)^(wrap) ^(—) ^(thresh)   Eq. (7)

Next, an error metric E_(h) is computed for each offset region. In oneexemplary embodiment, the error metric E_(h) is given by:E _(h)=(|

_(pusch)(i _(h)) −

_(pucch)(i _(h))|+⊕·|

_(pusch)(i _(h))−f′ _(pusch) ^(k−1)|)   Eq. (8)

The first term of the error metric is a difference between thehypothesized frequency offsets for the PUSCH and PUCCH. The second termis the difference between the hypothesized frequency offsets for thecurrent and preceding PUSCH subframes. The coefficient β is apredetermined weighting factor. Those skilled in the art will appreciatethat the use of the second term in Eq. 8 is optional. Next, the offsetregion that minimizes the error metric is found:

$\begin{matrix}{i_{h}^{\min} = {{\underset{i_{h} \in {\lbrack{0,1,2,\mspace{11mu}{\ldots\mspace{14mu} 9}}\rbrack}}{argmin}\left( E_{h} \right)}.}} & {{Eq}.\mspace{14mu}(9)}\end{matrix}$

The final frequency offset estimates are then given by:f′ _(pusch)=

_(pusch)(i _(h) ^(min))   Eq. (10)f′ _(pucch)=

_(pucch)(_(h) ^(min))   Eq. (11)

In PUSCH subframes without new PUCCH raw frequency offset estimates, anew hypothesized frequency offset is computed only for the PUSCH in eachoffset region according to Eqs. (4) and (5). Next, an error metric E_(h)is computed for each offset region. In one exemplary embodiment, theerror metric E_(h) is given by:E _(h)=(|

_(pusch)(i _(h))−f′ ^(k−1) _(pusch)|).   Eq. (12)

Next, the offset region that minimizes the error metric is foundaccording to Eq. (9). The new frequency offset estimate for the PUSCH isthen given by Eq. (10).

In new PUCCH subframes, new hypothesized frequency offsets may becomputed for each offset region i_(h) according to Eqs. (6) and (7).Next, an error metric E_(h) is computed for each offset region. In oneexemplary embodiment, the error metric E_(h) is given by:E _(h)=(|

_(pucch)(i _(h))−f′ ^(k−1) _(pucch)|).   Eq. (13)

Next, the offset region that minimizes the error metric is foundaccording to Eq. (9). The new frequency offset estimate for the PUCCH isthen given by Eq. (11).

In the procedure described above, the frequency ambiguity is resolved inPUSCH subframes with new PUCCH raw frequency offset estimates. In thesesubframes, the error metric takes into account the differences betweenthe hypothesized frequency offsets for the PUSCH and PUCCH. Thehypothesized frequency offsets in the offset region that minimizes theerror metric are likely to be the correct estimates.

The general procedure described above works with frequency offsets up to7000 Hz. In LTE applications, the expected frequency offset is expectedto be much lower than 7000 Hz. With a smaller frequency offset, themaximum wrap-around number is smaller, resulting in a smaller hypothesistable. Fewer hypotheses reduce complexity and improve the algorithmrobustness. For a band 7 frequency, it can be assumed that the maximumfrequency offset is below 2000 Hz. Thus, the hypothesis table reduces tothree offset regions. The computation of hypothesized frequency offsetsfor each offset region simply to:

For the PUSCH

_(pusch)(0)={circumflex over (f)}_(pusch)   Eq. (14)

_(pusch)(1)={circumflex over (f)} _(pusch) −sgn({circumflex over (f)}_(pusch))·2000   Eq. (15)

_(pusch)(2)={circumflex over (f)} _(pusch−) sgn({circumflex over (f)}_(pusch))·2000   Eq. (16)

For the PUCCH

_(pucch)(0)={circumflex over (f)} _(pucch)   Eq. (17)

_(pucch)(1)={circumflex over (f)} _(pucch)   Eq. (18)

_(pucch)(2)={circumflex over (f)} _(pucch) −sgn({circumflex over (f)}_(pucch))·3500   Eq. (19)

In PUSCH subframes with new PUCCH raw frequency estimates, newhypothesized frequency offsets are computed according to Eqs. (14)-(19).An error metric for each offset region is then computed according to Eq.(8) and the offset region that minimizes the error metric is selectedaccording to Eq. (9). The final frequency offset estimates for the PUSCHand PUCCH are then given by Eqs. (10) and (11), respectively.

In PUSCH subframes without new PUCCH raw frequency estimates, newhypothesized frequency offsets are computed only for the PUSCH accordingto Eqs. (14)-(16). An error metric for each offset region is thencomputed according to Eq. (12) and the offset region that minimizes theerror metric is selected according to Eq. (9). The final frequencyoffset estimate for the PUSCH is then given by Eq. (10).

In PUCCH subframes, new hypothesized frequency offsets are computed forthe PUCCH according to Eqs. (17)-(19). An error metric for each offsetregion is then computed according to Eq. (13) and the offset region thatminimizes the error metric is selected according to Eq. (9). The finalfrequency offset estimate for the PUCCH is then given by Eq. (11).

For band 1 frequency, the maximum frequency offset may be assumed to bebelow 1750 Hz. In this case, there is no wrap-around in the PUCCHfrequency offset estimates and the hypothesis table reduces to twooffset regions. The hypothesized frequency offsets for the PUSCH aregiven by Eqs. (14) and (15). The hypothesized frequency offsets for thePUCCH are given by Eqs. (17) and (18).

In PUSCH subframes with new PUCCH raw frequency estimates, newhypothesized frequency offsets are computed according to Eqs. (14)-(15)and Eqs. (17)-(18). An error metric for each offset region is thencomputed according to Eq. (8) and the offset region that minimizes theerror metric is selected according to Eq. (9). The final frequencyoffset estimates for the PUSCH and PUCCH are then given by Eqs. (10) and(11) respectively.

In PUSCH subframes without new PUCCH raw frequency estimates, newhypothesized frequency offsets are computed only for the PUSCH accordingto Eqs. (14)-(15). An error metric for each offset region is thencomputed according to Eq. (12) and the offset region that minimizes theerror metric is selected according to Eq. (9). The final frequencyoffset estimate for the PUSCH is then given by Eq. (10).

In PUCCH subframes, new hypothesized frequency offsets are computed forthe PUCCH according to Eqs. (17)-(18). An error metric for each offsetregion is then computed according to Eq. (13) and the offset region thatminimizes the error metric is selected according to Eq. (9). The finalfrequency offset estimate for the PUSCH is then given by Eq. (11).

FIG. 5 illustrates a generalized form of the joint estimation procedure200 as herein described. The procedure 200 shown in FIG. 5 begins withthe generation of raw frequency offset estimates for first and secondchannels having different resolvable ranges (block 210). Hypothesizedfrequency offsets are then computed from the raw frequency offsetestimates for each one of two or more offset regions in an offset rangeof interest (block 220). For each offset region, an error metric iscomputed as a function of the corresponding hypothesized frequencyoffsets for the first and second channels (block 230). The set ofhypothesized frequency offsets that minimizes the error metric is thenselected as the final frequency offset estimates (block 240).

The embodiments described herein expand the resolvable frequency offsetrange, which allows for more accurate estimation of the frequency offsetunder HST conditions in LTE. More accurate frequency offset estimationpaves the way for proper frequency offset compensation, thus ensuring acertain level of throughput performances. The embodiments describedherein require very little extra computing resource to implement.

The present invention may, of course, be carried out in other specificways than those herein set forth without departing from the scope andessential characteristics of the invention. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A method of determining a frequency offsetbetween a transmitting station and a receiving station, the methodcomprising: computing, for each of two or more offset regions within anoffset range of interest, a first hypothesized frequency offset for afirst channel and a second hypothesized frequency offset for a secondchannel wherein the first and second channels have different resolvableranges; computing, for each offset region, an error metric as a functionof the corresponding hypothesized frequency offsets for the first andsecond channels; and using, at a receiver within a wireless network, asfinal frequency offset estimates, the hypothesized frequency offsetscorresponding to the offset region having the lowest error metric. 2.The method of claim 1 wherein computing the first and secondhypothesized frequency offsets for each one of two or more offsetregions within a region of interest comprises: determining, for eachchannel, a wrapping threshold; determining from the wrapping thresholdsthe offset regions and corresponding wrap-around numbers for the firstand second channels such that the combination of wrap-around numbers forthe first and second channels is unique in each offset region; andcomputing, for each offset region, the hypothesized frequency offsetsfor the first and second channels based on the wrapping thresholds andthe number of wraps.
 3. The method of claim 1 wherein computing theerror metric for each offset region comprises: computing a firstdifference between the hypothesized frequency offsets for the first andsecond channels in a first time interval; and computing the error metricas a function of the first difference.
 4. The method of claim 3 whereincomputing the error metric for each offset region further comprises:computing a second difference between the hypothesized frequency offsetfor the first channel in the first time interval and a final frequencyoffset for the first channel in a second time interval prior to thefirst time interval; and computing the error metric as a weighted sum ofthe first and second differences.
 5. The method of claim 3 furthercomprising: computing, for a second time interval, hypothesizedfrequency offsets for the first channel for each one of two or moreoffset regions within a region of interest; computing, for the secondtime interval, a second error metric from the hypothesized frequencyoffsets for the first channel in the second time interval and a finalfrequency offset estimate for the first channel in the first timeinterval; and selecting, for the second time interval, the hypothesizedfrequency offset with the lowest second error metric.
 6. The method ofclaim 3 further comprising: computing, for a second time interval,hypothesized frequency offsets for the second channel for one or moreoffset regions within a region of interest; computing, for the secondtime interval, a second error metric from the hypothesized frequencyoffsets for the second channel in the second time interval and a finalfrequency offset estimate for the second channel in the first timeinterval; and selecting, for the second time interval, the hypothesizedfrequency offset with the lowest second error metric.
 7. The method ofclaim 1 in a Long Term Evolution (LTE) network wherein the first channelcomprises the Physical Uplink Shared Channel and the second channelcomprises the Physical Uplink Control Channel.
 8. The method of claim 1in a Long Term Evolution (LTE) network, wherein the first channelcomprises the Physical Uplink Shared Channel (PUSCH) and the secondchannel comprises the Physical Uplink Control Channel (PUCCH).
 9. Areceiving node in a mobile communication network, said network nodecomprising: a receiver front end for receiving signals transmitted froma transmitting node; and a processing circuit connected to said receiverfront end to estimate a frequency offset between the receiving node anda transmitting node, said processing circuit comprising a frequencyoffset estimator configured to: compute, for each of two or more offsetregions within a range of interest, a first hypothesized frequencyoffset for a first channel and a second hypothesized frequency offsetfor a second channel wherein the first and second channels havedifferent resolution ranges; compute, for each offset region, an errormetric as a function of the corresponding hypothesized frequency offsetsfor the first and second channels; and select, as final frequency offsetestimates, the hypothesized frequency offsets corresponding to theoffset region having the lowest error metric.
 10. The receiving node ofclaim 9 wherein the frequency offset estimator is configured to computethe first and second hypothesized frequency offsets for each one of twoor more offset regions within a region of interest by: determining, foreach channel, a wrapping threshold; determining from the wrappingthresholds, the offset regions and corresponding wrap-around numbers forthe first and second channels such that the combination of wrap-aroundnumbers for the first and second channels is unique in each offsetregion; and computing, for each offset region, the hypothesizedfrequency offsets for the first and second channels based on thewrapping thresholds and the number of wraps.
 11. The receiving node ofclaim 9 wherein said frequency offset estimator is configured to computethe error metric for each offset region as a function of thehypothesized frequency offsets for the first and second channels by:computing a first difference between the hypothesized frequency offsetsfor the first and second channels in a first time interval; andcomputing the error metric as a function of the first difference. 12.The receiving node of claim 11 wherein the frequency offset estimator isconfigured to compute an error metric for each offset region by:computing a second difference between the hypothesized frequency offsetfor the first channel in the first time interval and a final frequencyoffset for the first channel in a second time interval prior to thefirst time interval; and computing the error metric as a weighted sum ofthe first and second differences.
 13. The receiving node of claim 11wherein the frequency offset estimator is configured to compute theerror metric for each offset region by: computing, for a second timeinterval, hypothesized frequency offsets for the first channel for eachone of two or more offset regions within a region of interest;computing, for the second time interval, a second error metric from thehypothesized frequency offsets for the first channel in the second timeinterval and a final frequency offset estimate for the first channel inthe first time interval; and selecting, for the second time interval,the hypothesized frequency offset with the lowest second error metric.14. The receiving node of claim 11 wherein the frequency offsetestimator is configured to compute the error metric for each offsetregion by: computing, for a second time interval, hypothesized frequencyoffsets for the second channel for one or more offset regions within aregion of interest; computing, for the second time interval, a seconderror metric from the hypothesized frequency offsets for the secondchannel in the second time interval and a final frequency offsetestimate for the second channel in the first time interval; andselecting, for the second time interval, the hypothesized frequencyoffset with the lowest second error metric.
 15. The receiving node ofclaim 9 in a Long Term Evolution (LTE) network wherein the first channelcomprises the Physical Uplink Shared Channel and the second channelcomprises the Physical Uplink Control Channel.
 16. The receiving node ofclaim 9 configured for a Long Term Evolution (LTE) network wherein thewherein the first channel comprises the Physical Uplink Shared Channel(PUSCH) and the second channel comprises the Physical Uplink ControlChannel (PUCCH).